The present invention relates to the field of LED based lighting and more particularly to a constant current source for a series LED string having a voltage control feedback.
Light emitting diodes (LEDs) and in particular high intensity LED strings are rapidly coming into wide use. High intensity LEDs are sometimes called high power LEDs, high brightness LEDs, high current LEDs or super luminescent LEDs and are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a monitor. In a large LCD monitor typically the high intensity LEDs are supplied in a string of serially connected high intensity LEDs, thus sharing a common current. The term LED as used herein is meant to include any LED used to generate a light output and is meant to include, without limitation, any and all of high intensity LEDs, high power LEDs, high brightness LEDs, high current LEDs and super luminescent LEDs.
In order supply a white backlight for the monitor one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique individual strings of colored LEDs are placed in proximity so that in combination their light is seen a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are typically located at one end, one side, or in the back of the monitor, the light being diffused to appear behind the LCD by a diffuser. In the case of colored LEDs, a further mixer is required, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The mixer may be integrated within the diffuser. The white point of the light is an important factor to control, and much effort in design in manufacturing is centered on the need for a correct white point.
Each of the colored LED strings is typically intensity controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the diode string to a value achieved as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed, is held constant to maintain the white point among the disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight. The PWM may be further adjusted during operation to correct for any color imbalance caused by temperature or aging of the colored LEDs.
Each of the disparate colored LED strings has a voltage requirement associated with the forward drop and number of colored high intensity LEDs of the LED string. In one prior art method, a linear regulator per LED string is used to maintain a constant current. Unfortunately, excess power dissipation in the regulator results in an overall inefficient circuit, particularly if the voltage is unregulated and varies over a wide range.
U.S. Pat. No. 6,369,525 issued Apr. 9, 2002 to Chang et al, the entire contents of which is incorporated herein by reference, is addressed to a secondary side post regulator for use with LED arrays. The circuit comprises a plurality of secondary controllers, each associated with a particular secondary winding and configured to control a flow of current to its respective LED array. Unfortunately, Chang does not teach the use of PWM to achieve an overall luminance in cooperation with the secondary side post regulator controllers. The use of PWM leads to significant voltage output transients which results in distorted LED current waveforms causing significant inaccuracy in color and luminance of LCD monitors.
There is thus a long felt need for a voltage controlled source, preferably implemented in a secondary side post regulator, which is adapted for use with PWM switched current loads.